Question

ABO blood grouping is controlled by gene I which has three alleles and shows codominance. There are six genotypes. How many phenotypes in all are possible?
A. Five
B. Six
C. Three
D. Four

Answer 1

Genotype is the genetic constitution of an organism. On the other hand, the phenotype is the physical appearance of any organism. The ratio of the genotype to the phenotype in the case of the blood groups is $3:2$.

Complete answer: The Human ABO blood grouping system is a very good example of multiple allelism. The ABO blood groups are controlled by gene I. Gene I has three alleles. These three alleles are as follows –
1. $I^A$
2. $I^B$
3. $I^O$ or i
Now multiple alleles are a feature of the population. This means that all these three alleles are seen in a population only and not in an individual. In an individual, we can only see any two alleles. Thus, due to these three alleles, the total number of possible genotypes is six. This is because out of three objects when two are taken at a time, then the total number of permutations is six. These six genotypes are as follows –
1. ${I^A}{I^A}$
2. ${I^A}{I^O}$
3. ${I^B}{I^B}$
4. ${I^B}{I^O}$
5. ${I^A}{I^B}$
6. ${I^O}{I^O}$ or ii
Now phenotype is the physical expression of an organism. The phenotype for the six genotypes will be as follows –
i. ${I^A}{I^A}$ - A
ii. ${I^A}{I^O}$- A
iii. ${I^B}{I^B}$- B
iv. ${I^B}{I^O}$- B
v. ${I^A}{I^B}$- AB
vi. ${I^O}{I^O}$ or ii – O
The different phenotypes thus obtained are A, B, AB, and O. The blood group AB is an example of codominance. Here both the alleles express themselves equally without their expressions mixing together. Therefore, the three alleles and six genotypes show four different phenotypes. Thus, based on the above information we can conclude that ABO blood grouping is controlled by gene I which has three alleles and shows codominance. There are six genotypes and four phenotypes.
Hence, the correct answer is option (D).

Note: Multiple allelism refers to the presence of more than two alleles for a gene. According to Mendel, every gene has two contrasting forms. However, some genes have more than two contrasting forms or alternative forms. This alternative or contrasting form is known as the alleles and this phenomenon is known as multiple allelism. Multiple alleles are always seen in a population and not in an individual.